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What about bosses though? Or were they just "very hard mobs" with their own very top lvl gear? And I'm assuming the difficult mobs had a long spawn cd, or were they so difficult that even groups of people could only kill them once in a while?
You've now reminded me of partially why FFXI does not have the same itemization concept as much.
Top level bosses drop certain gear items, but the alternate way of getting them is through an instance. Whenever you kill any mob that gives exp (up to 10 levels below you at most levels, a bit more as you go higher), you get a chance for a 'Seal' to drop. That seal cannot be traded to other players. Trade some number of these 'Seals' to an NPC for an Orb to enter an instanced battlefield. Win that battlefield and get SOMETHING, always something, but certain things had 10% odds of dropping on average.
A lot of it was gear for people playing around with specialized builds, stuff you might use or give to a friend who happened to care (or for running the same sort of level-capped content), and at the higher levels of them (takes more Seals to unlock), you get chances at good items for top level crafting to make the better gear in the game.
So TECHNICALLY you could get a group together and blitz through 30 of these in a day if you saved up enough seals... but normally for my group at least they're just 'alright time for a break from regular play let's go do something interesting like die to an enemy Beastman group', if we win we make some money, if we lose we got a fight. But we had control of the when and in some ways the 'how-many', just it was balanced against the 'how-often'.
The thing is, though, because it was 'slower', a lot of people never fought these at all, and to 'have them figured out' took... well, the above. This system is entirely separate from the 3-day respawns and 1-week respawns of High Notorious Monsters (basically Raid Level at the time), but you could, with enough high level Seals, fight an instanced equivalent with similar drops with 18 people.
So while they didn't release new versions of these often, you'd get them in bulk, and over the next few weeks people would trickle out information on each one, as different people used up their 'Seals' to try the fights.
tl;dr fighting enough normal enemies opened up Instanced enemies that you could put on your own schedule, but they have a time limit and you get nothing if you lose.